This invention relates to testing of semiconductor dice contained on a wafer. More particularly, this invention relates to an improved probe card, and to a test method and test system employing the probe card.
Semiconductor wafers are tested prior to singulation into individual die, to assess the electrical characteristics of the integrated circuits contained on each die. A typical wafer-level test system includes a wafer handler for handling and positioning the wafers, a tester for generating test signals, a probe card for making temporary electrical connections with the wafer, and a prober interface board for routing signals from the tester pin electronics to the probe card.
The test signals can include specific combinations of voltages and currents transmitted through the pin electronics channels of the tester to the prober interface board, through the probe card, and then to one or more devices under test on the wafer. During the test procedure response signals such as voltage, current and frequency can be analyzed and compared by the tester to required values. The integrated circuits that do not meet specification can be marked or mapped in software. Following testing, defective circuits can be repaired by actuating fuses (or anti-fuses) to inactivate the defective circuitry and substitute redundant circuitry.
One type of probe card includes needle probes for making temporary electrical connections with die contacts on the dice (e.g., bond pads, fuse pads, test pads). The probe card typically includes an insulating substrate, such as a glass filled resin. The substrate can include electric traces in electrical communication with the needle probes. In addition, the needle probes can be configured to make electrical connections with a specific die, or groups of dice, on the wafer. Typically, the wafer or the probe card is stepped so that the dice on the wafer are tested in sequence.
One aspect of these testing procedures, is that the die contacts on the wafer are typically coated with a metal oxide layer. For example, aluminum bond pads can be covered with an aluminum oxide layer that forms by oxidation of the underlying metal. The oxide layer is electrically non conductive, and provides a high degree of electrical resistance to the needle probes. In order to ensure accurate test results, the needle probes must penetrate the oxide layer to the underlying metal.
To penetrate oxide layers on die contacts, the probe card and wafer can be brought together until the needle probes touch the die contacts. The probe card can then be overdriven a distance in the z-direction (e.g., 3 mils) causing the needle probes to bend. As the needle probes bend, their tips move horizontally across the die contacts, scrubbing through the oxide layers to the underlying metal. This scrubbing action also displaces some of the underlying metal causing grooves and corresponding ridges to form on the die contacts.
Due to their inherently fragile nature and mode of use, probe cards with needle probes require a significant amount of cleaning, alignment, and replacement. Another shortcoming of needle probe cards, is the needle probes cannot be fabricated with a density which permits testing of high pin count devices having dense arrays of die contacts. For example, fabricating probe needles with a pitch of less than about 6 mils has been difficult.
In view of the foregoing, it would be advantageous to provide a probe card which is capable of accurately probing dense arrays of closely spaced die contacts on semiconductor wafers. It would also be advantageous to provide a probe card which is relatively simple to construct and maintain. Still further, it would be advantageous for a probe card to include contacts that are more robust than needle probes, and which can move in a z-direction to accommodate penetration of oxide layers on the die contacts.
In accordance with the present invention, an improved probe card for testing semiconductor wafers is provided. Also provided are testing methods, and testing systems employing the probe card. The probe card, simply stated, comprises: a substrate; patterns of pin contacts slidably mounted to the substrate; and a force applying member for biasing the pin contacts into electrical engagement with die contacts on the wafer.
The substrate comprises a material such as ceramic, silicon, photosensitive glass-ceramic, or glass filled resin, which can be etched, machined or molded with patterns of densely spaced openings. The pin contacts are mounted within the openings, one pin contact per opening. The size and pitch of the openings and pin contacts, permits testing of dice having dense arrays of die contacts. In addition, the pin contacts are capable of Z-direction axial movement within the openings, to permit oxide layers on the die contacts to be penetrated, and to accommodate differences in the vertical locations of the die contacts. The pin contacts can include pointed tip portions for penetrating the die contacts, and enlarged head portions for making electrical connections with the pin contacts.
The force applying member applies a biasing force to the pin contacts. The biasing force can be controlled such that the pin contacts penetrate the die contacts to a limited penetration depth. Several different embodiments for the force applying member are provided. In a first embodiment, the force applying member comprises spring loaded electrical connectors, such as xe2x80x9cPOGO PINSxe2x80x9d, which engage the head portions of the pin contacts. The electrical connectors mount to a support plate attached to a test head wherein the probe card is mounted. In this embodiment the electrical connectors provide both the biasing force, and the external electrical connections to the pin contacts. A biasing force exerted by the electrical connectors is dependent on a spring constant, and compression, of a spring component of the electrical connectors. In addition, the biasing force is dependent on the amount by which the pin contacts are overdriven into the die contacts past the initial point of contact.
In a second embodiment, the force applying member comprises a compressible pad mounted to the substrate for biasing the head portions of multiple pin contacts. In this embodiment a separate interconnect layer, such as a flex circuit, can be provided for fanning out electrical paths from the pin contacts, to accommodate larger pitches for electrical connections to the pin contacts.
In a third embodiment, the openings in the substrate have closed ends, and the force applying member comprises separate compressible pads mounted within the openings in physical contact with the pin contacts. In this embodiment the openings can include internal contacts which make electrical connections to the pin contacts and to the outside. Alternately, the compressible pad can comprise an electrically conductive polymer which physically and electrically contacts the pin contacts.
A test system constructed in accordance with the invention includes the probe card mounted to a conventional wafer handler. The wafer handler is adapted to align the pin contacts on the probe card to the die contacts on the wafer, and to bring the wafer and probe card in contact with a desired amount of Z-direction overdrive of the pin contacts into the die contacts. The test system also includes a tester in electrical communication with the probe card. The tester is adapted to transmit test signals through the probe card to the dice contained on the wafer, and to analyze resultant test signals.